Viscous damper using magnetic ferrofluid



NOV. 3, 1970 LI ET AL 3,538,469

vxscous DAMPER USING MAGNETIC 'FERROFLUID Filed Sept, 22, 1967 INVENTORS. RUDOLPH LITTE BY LEO BELTRACCHI M hip '7' IF-L Gw M ATTORN Y5.

United States Patent Oflice 3,538,469 Patented Nov. 3, 1970 US. Cl. 335-219 9 Claims ABSTRACT OF THE DISCLOSURE A ferrofluid is contained and hermetically sealed Within a rotatably mounted annular chamber, and is captured and held by a magnetic field. In one embodiment the magnetic field is stationary, and the viscous shear forces established by the surfaces of the annular chamber and the stationary captured fluid cause damping of the rotation or oscillation of the surface. In a second embodiment the magnetic field which captures the ferrofluid is rotating and causes the magnetic fluid to rotate. The viscous shear forces between the surfaces of the annular chamber and the rotating captured fluid produce rotation of the chamber.

BACKGROUND OF THE INVENTION All known strongly magnetic materials are solids, and no true ferro-magnetic liquids are presently available. However, it has recently been found possible to make a stable suspension of finely divided ferro-magnetic solids in a carrier liquid, to produce a fluid that reacts strongly when a magnetic field is applied to it. Such material has been called ferrohydrodynamic fluid (FHD), magnetic fluid, or a ferrofluid. In this application the term ferrofluid will be used.

Dispersions of exceedingly fine colloidal magnetic particles can be produced by grinding ferro-magnetic powders, such as ferrites, with steel balls in a tumbling mill for several weeks. The grinding is done in the presence of a liquid carrier, such as kerosene, which has dissolved in it a dispersing agent, for example, oleic acid, to form a resilient layer on the surface of each particle. The typical particle size is 100 angstroms, and the particles are in agitation by impact of the liquid molecules (Brownian motion) which prevents settling. Thus, these colloidal mixtures behave mechanically like true homogeneous fluids but they have the characteristics of a magnetic fluid.

' While this invention requires a ferrofluid having particular characteristics, the composition and the method of manufacture form no part of the invention. For additional information concerning manufacture and composition, reference may be made to a paper entitled Ferrohydrodynamic Fluids for Direct Conversion of Heat Energy by Rosensweig, Nestor and Timmins appearing in the A.I. Ch.E.-I. Chem. joint meeting, June 1965, and a paper entitled The Fascinating Magnetic Fluids by Rosensweig appearing in the New Scientist 29, No. 479, Jan. 20, 196 6.

CHARACTERISTICS OF THE FERROFLUID Two ferrofluid characteristics are essential to this invention. First, the ferrofluid must be susceptible to capture by a magnetic field. Second, the fluid must have sutficient viscosity for its use either as a damper or as a coupling device. In addition, certain ferrofluids do demonstrate a levitation phenomenon which makes certain variations of this invention possible. In accordance with the levitation phenomenon, a non-magnetic object immersed in ferrofluid will drift to a point of minimum field to be held in stable equilibrium as if by a three-dimensional spring.

THE DRAWINGS FIG. 1 schematically illustrates a damping mechanism constructed in accordance with the teachings of this invention; and

FIG. 2 represents a coupling drive mechanism utilizing the principles of this invention.

DESCRIPTION OF THE FIG. 1 EMBODIMENT The purpose of the damping mechanism shown in FIG. 1 is to provide an energy dissipative mechanism for a dynamic system. The structure illustrated includes a shaft 10 rotatably supported between bearings 12 and 14. The shaft may be freely rotatable or it may be supported by a free-flex pivot which biases the shaft to a predetermined nominal angular position. In a very simple application the shaft may carry a plumb, it being the object of the damping mechanism to damp out oscillations rapidly.

A hollow damper wheel 16 is fixedly mounted concentrically on the shaft 10 by means of a spoke 18. The wheel 16 is fabricated from a non-magnetic material, such as aluminum, plastic, etc. A ferrofluid 20 is contained within the annular chamber 22 formed by the hollow damper wheel 16, and a magnet 24, having its pole faces 26 and 28 positioned adjacent opposite sides of the wheel, is fixed with respect to the axis of the shaft. The magnet 24 may be a permanent magnet or an electromagnet, but in any case must be powerful enough to capture the ferrofluid 20. That is, if the damper wheel 16 is oscillating, the magnet 24 holds the ferrofluid stationary, overcoming forces of gravity, inertial reaction forces, and viscous shear forces.

The magnet structure is attached to one element of a dynamic system, while the wheel assembly is attached to another element of the dynamic system. Relative motion between the two elements is damped because of energy dissipation due to the viscous shear forces. This energy dissipation is proportional to the relative velocity between the two elements of the dynamic system. Therefore, this invention provides a viscous damper which functions as a proportional damper.

The primary advantage of the damping system is its utter simplicity. Because the ferrofluid may be hermetically sealed within the annular wheel, and because of the absence of any mechanical coupling between the two elements of the dynamic system, this damping device should work indefinitely under many varied conditions.

The invention accommodates many modifications. For example, different types of magnetic structures can be used to permit capturing of large volumes of fluid without increase in size of the magnetic structure. This may be accomplished by using two magnets, one on each boundary of the ferrofluid. In another arrangement the remainder of the chamber 22 could be filled by adding an immiscible fluid to the ferrofluid. This would provide additional shear force since the immiscible fluid would be held stationary by the captured ferrofluid.

Other variations could be incorporated by having sev eral annular chambers effectively operating in parallel. This would increase the shear area without significantly increasing the magnet size.

In summary, the damper makes use of a unique magnetic field which is completely sealed within an annular chamber which may take the form of a hollow wheel or a disk. A permanent magnet attracts the magnetic fluid and captures it. Shearingof the fluid and viscous friction forces result whenever the damper wheel moves with respect to the permanent magnet. These shearing and viscous shearing forces damp the relative motion.

This type of damper has proved to be extremely insensitive to parameter variations, and the optimum viscous 3 damping coeflicient does not change appreciably for various conditions of operation.

In summary, the concept involved provides that a volume of a magnetic fluid is captured and held by a permanent magnetic field. A surface moving through the fluid is subject to a viscous shear force. Where the relative velocities are very small, the damping action is pure vis cous (proportional to velocity). Furthermore, since the magnetic fluid only partially fills the damper wheel, temperature expansion is not a problem. Furthermore, since only the disk cavity contains fluid, there are no seals to be concerned with, and the fluid chamber is sealed hermetically.

DESCRIPTION OF THE FIG. 2 EMBODIMENT While the arrangement of FIG. 1 serves as a damper, the same principles may also be useful as a coupling mechanism. In the embodiment of FIG. 2 a shaft 30- is rotatably mounted in bearings 32 and carries a rotatable hollow wheel 34 supported from a fixedly mounted spoke 36. The hollow wheel 34, which could take the form of a disk, is partially filled with a ferrofluid 38. A second shaft 40, rotatably mounted in bearings 42, carries a second hollow wheel 44 fixedly mounted by means of a spoke 46. Attached within the hollow wheel 44 is a permanent magnet 48. The shaft 40 is driven by any suitable power source 50.

As in the previous embodiment, the magnet 48 must be of a size sutficient to capture the ferrofluid 38. Under these circumstances when the magnet 48 is rotated, the ferrofluid 38 within the hollow wheel 34 is also rotated. The inertial forces, the viscous friction forces, and the shear forces existing between the surfaces of the hollow Wheel 34 and the ferrofluid 38 cause the wheel 34 and the shaft 30 to rotate. Suitable output coupling means may, of course, be connected either to the wheel 34 or a portion of the shaft 30.

Because the viscous shear forces are proportional to velocity, essentially no coupling exists at zero relative velocity between the wheel 34 and the ferrofluid 38. Therefore, it will be recognized that there will be some relative velocity between the wheels 34 and 44.

The invention is susceptible to various modifications and adaptations, and it is intended, therefore, that this invention will be limited only by the appended claims as interpreted in the light of the prior art.

What is claimed is:

1. A viscous coupling device comprising an annular non-magnetic container rotatable on its axis;

a ferrofluid partially filling said container, said ferrofluid being viscous, and having magnetic properties; and

a magnet mounted adjacent said container externally thereof, the magnetic field of said magnet capturing said ferrofluid to prevent relative rotation between said ferrofluid and said magnet, whereby relative rotation of said magnet and said container produces viscous shearing between said ferrofluid and the inner surfaces of said container.

2. The invention as defined in claim 1 wherein said magnet is fixedly mounted with respect to the axis of said container.

3. The invention as defined in claim 2 wherein said partially filled container is hermetically sealed.

4. The invention as defined in claim 3 wherein said container is in the form of a hollow wheel, said hollow wheel being fixed to a rotatable shaft.

5. The invention as defined in claim 4 wherein said magnet is a permanent magnet.

6. The invention as defined in claim 1 wherein said magnet is rotatably mounted on an axis coincident with the axis of said container.

7. The invention as defined in claim 6 wherein said partially filled container is hermetically sealed.

8. The invention .as defined in claim 7 wherein said container is in the form of a hollow wheel, said hollow wheel being fixed to a rotatable shaft.

9. The invention as defined in claim 8 wherein said magnet is a permanent magnet.

References Cited UNITED STATES PATENTS 3,250,341 5/1966 Takahashi 19221.5 XR 3,106,850 10/1963 Clisset 19221.5 XR 2,987,153 6/1961 Perry 192-215 2,996,267 8/1961 Warren 19221.5 XR 2,575,360 11/1951 Rabinow 310-92. XR

BERNARD A. GILHEANY, Primary Examiner D. M. MORGAN, Assistant Examiner US. Cl. X.R. 335296; 192-215 

